In the United States, federal regulations require that poultry products be chilled soon after slaughter and evisceration. While the regulations have recently been rewritten to provide the processor more flexibility, the previous prescriptive requirement is still presumed to be sufficient and provides a useful frame of reference for the process. The previous regulation required the internal bird temperature to be reduced to 40° F. or less within 4 to 16 hours of slaughter depending on bird weight and chilling method. Some processors adopt a lower target temperature in order to achieve commercial goals such as improved shelf life. This invention addresses reducing the temperature of products that may be whole eviscerated birds or any part thereof or bird giblets or other non-poultry products such as pork carcasses or beef tongues.
Commercial considerations also drive processors to effect chilling in a shorter timeframe than the regulatory guidance. In general, longer chilling times require more product to be resident in the chiller, and consequently the equipment must be larger. Not only is larger equipment more costly, it takes up more room in the plant and consequently increases the facility cost.
In addition to capital considerations, longer chilling times also complicate operations. Processors must schedule operations that occur before chilling such as evisceration and operations after chilling such as cut-up with sufficient delay for the chilling operation. Some processing facilities are on a 24 hour cycle with two shifts for processing followed by cleanup on third shift. For such operations, there is no more time available in the day to allocate for chilling.
There has been a sustained trend in the poultry industry toward increasingly large birds. Birds in excess of 10 pound live weight are now quite common. Due to the thickness of the meat, these large birds take longer to chill than smaller birds would in the same chilling equipment. This trend has created incentives for new technology that can reduce the chilling time.
Consider first the conduction of heat from the interior of the product (e.g., carcass) to the surface. It is well known that the conductive heat transfer rate between two points in a solid body is generally proportional to the difference in temperature between those points. Thus, the heat transfer rate from, for example, a point inside the breast muscle to the skin adjacent this muscle is proportional to the difference in temperature between those places. The actual rate further depends upon the thermal conductivity of the material (in this case muscle tissue) as well as the specific heat and mass density since the process is transient. The initial temperature at the center of the muscle is about 104° F. which is the nominal body temperature of chickens. Neither the initial temperature nor the thermal properties of the product can be altered. Thus, the only way to increase heat transfer and reduce the chilling time is to reduce the temperature at the surface of the product. This part of the theoretical problem is common to all chilling methods.
The method used for chilling the product determines how heat is transferred away from the surface of the product. Two chilling methods are well documented in the prior art: immersion chilling and air chilling.
The immersion chilling method causes the hot product to be submerged in chilled water or a mixture of water and ice. In the chiller, heat is removed from the surface of the product by convection of the chilling medium. Heat moves from the interior of the meat to the surface by conduction. The product remains in the chiller for a sufficient length of time so that the interior temperature is reduced to the target level. Over time, a variety of methods have been developed for controlling the length of time individual products remain immersed and these methods have come to be recognized as chiller types such as rocker chillers, drag chillers and auger chillers.
Within each type of immersion chiller, various techniques have evolved for increasing the convective heat transfer coefficient between the product surface and the body of water. However, even at very high convective coefficients, the surface temperature of a product in an immersion chilling system cannot be less than 32° F., since the water would freeze below that point and cannot be circulated about the product.
This inherent limitation of immersion chilling is significant, because in many cases the target for the interior temperature is only 36° F. This provides a temperature difference of only 4° F. within the product during the final stages of chilling. Such a small differential slows the chilling process.
The air chilling method sees individual products suspended in a chamber in which cold air is circulated around the product. Air does not suffer the temperature limitation of water; however, the convective heat transfer coefficient achievable with air is much lower than for water. Further, if the air temperature is too low, exposed parts of the product such as wings may freeze before heavier parts such as breast meat gets cold. In general, air chilling takes longer than immersion chilling.
It should be noted that a substantial part of the convective heat transfer in air chilling results from evaporation of water from the surface of the product.
In both immersion chilling and air chilling, heat transfer from the inner surface of the product—for instance the surface facing the organ cavity in a carcass—is limited by the difficulty of circulating the chilling medium through this cavity.
The present invention introduces the use of vacuum chilling to remove heat from the surface of product. The primary mode of heat transfer is evaporation at the surface. Vacuum chilling does not suffer the temperature limits of immersion chilling and enjoys higher heat transfer rates than air chilling.
There is no prior art known to the inventor related to using vacuum chilling for poultry or other meat products. Vacuum chilling has been used to remove field heat from certain fruits and vegetables to improve quality and extend shelf life. Typically, the process is operated in batch mode in which palletized produce is loaded into vacuum chambers which are sealed and then evacuated. Pressure is held at a very low level for a period of time, and then air is allowed back into the chamber and the produce is removed. Operating pressures are reduced to the flash point where water boils at the temperature of the product. This type of operation is well suited for products that are already batched into boxes and pallets. Also, the equipment is usually portable for moving from farm to farm as the growing season progresses.
Vacuum chilling is also used for other applications such as making ice.